Method for making doped yttrium oxide phosphors



Unite 3,291,747 Patented Dec. 13, 1966 3,291,747 METHQD FOR MAKING DOPEDYTTRIUM ()XIDE PHUSPHGRS Robert A. Lct'ever, Albuquerque, N. Mean, andKenneth A. Wiclcersheim, Menlo Park, Califi, assignors to GeneralTelephone and Electronic Laboratories, Inc, a corporation of Delaware NoDrawing. Continuation of application Ser. No. 213,896, Aug. 1, 1962.This application Mar. 31, 1965,

Ser. No. 444,460

4 Claims.

Our invention relates to phosphors and methods for making same.

This application is a continuation of application Serial No. 213,896,filed August 1, 1962, now abandoned.

We have invented a new class of phosphors which, as compared to knownphosphors, exhibits significantly enhanced fluorescence when excited bybombardment with electrically charged particles. Accordingly, ourinvention provides both a new class of phosphors of the characterindicated and methods for making the same.

These and other aspects of our invention will either be explained orwill become apparent hereinafter.

In accordance with the principles of our invention high purity yttriumoxide after heat treatment is found to exhibit blue-whitecathodoluminescence. When desired, a doping ion can be added to theoxide and the same or even enhanced cathodoluminescence will ensue.Typically, the doping ion can be, for example, cadmium, magnesium,copper or thorium.

Illustrative embodiments of our invention will now be described withreference to the specific examples which follow.

Example I 2 grams of high purity (99.999 percent pure) yttrium oxidepowder were heated to 1300 C. for a period of two hours. This powder wasapplied as a coating to the inner surface of a screen in a demountablecathode ray tube and was bombarded by an electron beam using a beamcurrent of 5 microamperes and a voltage of 1500 volts between the screenand the electron gun. Bright bluewhite cathodoluminescence was observed.When the voltage and current were increased to the maximum availablewith our equipment, 5000 volts and 20 microamperes respectively, nopermanent damage (i.e. burning) of the screen was observed.

This process was repeated, omitting the heat treatment of the oxidepowder, and extremely faint blue-white cathodoluminescence was observed.

The same process was repeated using higher heating temperatures, as forexample heating the powder to 2400 C. by passing the powder through anoxy-hydrogen flame. Blue-white cathodoluminescence was observed, thebrightness level being considerably higher than that produced by thepowder heated to 1300 C.

Example 11 2 grams of mechanically mixed powders composed of about 1mole percent of magnesium oxide and 99 mole percent of yttrium oxidewere weighed out into a 2000 milliliter beaker. 42 milliliters ofcommercial grade concentrated nitric acid were added to the beaker. Thesample was heated to a temperature falling Within the range 8090 C. fora period of -20 minutes until the oxide was dissolved. The solution wasthen diluted with 500 milliliters of distilled water, and the dilutesolution was heated to 80 C.

40 grams of oxalic acid were dissolved in 400 milliliters of distilledwater, and the oxalic acid solution was heated to a temperature of 70-80C. The hot oxalic acid solution as added to the hot dilute solutioncontaining magnesium and yttrium.

During the ensuing reaction, an oxalate precipitate formed and wasallowed to settle and cool. The solution was decanted, and theprecipitate was washed several times with a 2 percent oxalic acidsolution. After filtration with a Biichner funnel using whatman No. 50(or equivalent) filter paper, the filtrate was placed in a 50 milliliterplatinum crucible and was ignited over a burner for about 10 minutes.

The crucible was then loaded into a furnace and ignited at 850 C. forone hour to convert completely the oxalate into an oxide. The resultingcake was ground to powder in a mortar and pestle to produce a phosphorwith yttrium oxide as the host material and magnesium (present in anamount equal to 1 mole percent of cation concentration) as the dopingion.

This phosphor was tested in a demountable cathode ray tube as in ExampleI. Again, blue-white cathodoluminescence was observed, the brightnesslevel being about the same as that at the 1300 C. heated undoped yttriumoxide.

Example III A sample prepared by the process of Example II was heated to1300 C. for 2 hours. Improved blue-white cathodoluminescence wasobserved, the brightness level being about the same as that of the 2400C. heated undoped yttrium oxide.

Example IV The process of Example II was repeated with various dopingions (i.e. cadmium, copper and thorium) to produce the followingphosphors: yttrium oxide doped with 1 mole percent of cadmium; yttriumoxide doped with 1 mole percent of copper; and yttrium oxide doped with1 mole percent of thorium. All of these phosphors, when separatelytested in a demountable cathode ray tube in the manner previouslyindicated, were found to exhibit blue-white cathodoluminescence of abrightness comparable to that obtained in Example II.

Further experiments indicated that as the doping ion concentration wasincreased appreciably above about 1 mole percent, little improvement inthe cathodoluminescence brightness level was observed.

While we have shown and pointed out our invention as applied above, itwill be apparent to those skilled in the art that many modifications canbe made Within the scope and sphere of our invention.

What is claimed is:

1. A process for producing a cathodoluminescent phosphor which comprisesthe steps of (a) preparing a first hot acidic solution containingapproximately 99 mole percent yttrium oxide and approximately 1 molepercent of a dopant selected from the group consisting of magnesium,cadmium, copper and thorium,

(b) reacting said first solution with a second hot solution of oxalicacid,

(c) allowing the reactants to cool to produce an oxalate precipitate ofyttrium and a dopant selected from said group, and

(d) heating said oxalate precipitate to convert the oxalate to an oxideof yttrium incorporating an element from said group, the resultantphosphor exhibiting a blue-white cathodoluminescence.

2. A process for producing a cathodoluminescent phosphor which comprisesthe steps of (a) heating approximately 99 mole percent yttrium oxide andapproximately 1 mole percent of a dopant selected from the groupconsisting of magnesium, cadmium, copper and thorium with nitric acid toform a first dilute solution having a temperautre of approximately C.,

(b) preparing a second solution of oxalic acid and heating said solutionto a temperature in the range 70"- 80 C.,

(c) reacting said first and second solutions to form an oxalateprecipitate of yttrium and a dopant selected from said group, and

(d) heating said oxalate precipitate at a temperature in the range850-1300 C. for l to 2 hours to convert the oxalate to an oxide ofyttrium incorporating an element from said group, the resultant phosphorexhibiting a blue-White cathodoluminescence.

3. A process for producing a cathodoluminescent phosphor which comprisesthe steps of (a) mixing approximately 99 mole percent yttrium oxide andapproximately 1 mole percent magnesium oxide with nitric acid,

(b) heating said mixture to a temperature in the range 8090 C. until theoxide is dissolved to form a first solution,

() preparing a second solution and heating said solution of oxalic acidto a temperature in the range 70- 80 C.,

(d) reacting said first and second hot solutions to form an oxalateprecipitate of yttrium and magnesium, (e) allowing said precipitate tosettle and cool, and (f) heating said precipitate to 850 C. for aboutone hour to convert completely the oxalate into an oxide of yttrium andmagnesium, the resultant phosphor exhibiting a blue-Whitecathodoluminescence.

4. A process for producing a cathodoluminescent phosphor which comprisesthe steps of (a) mixing approximately 99 mole percent yttrium oxide andapproximately 1 mole percent magnesium oxide With nitric acid,

(b) heating said mixture to a temperature in the range 8090 C. until theoxide is dissolved to form a first solution,

(c) preparing a second solution and heating said solution of oxalateacid to a temperature in the range C.,

(d) reacting said first and second hot solutions to form an oxalateprecipitate of yttrium and magnesium,

(e) allowing said precipitate to settle and cool, and

(f) heating said'precipitate to 1300 C. for about two hours to convertcompletely the oxalate into an oxide of yttrium and magnesium, theresultant phosphor exhibiting a blue-white cathodoluminescence.

References Cited by the Examiner Partington: Textbook of InorganicChemistry, Macmillan and Co., Ltd., London, 1950, 6th edition, pages819- 20.

Pascal: Nouveau Traite de Chimie Minerale, Masson and Cie, Paris, 1959,Tome (VII), pages 189-91.

HELEN M. MCCARTHY, Acting Primary Examiner. TOBIAS E. LEVOW, Examiner.

R. EDMONDS, Assistant Examiner.

1. A PROCESS FOR PRODUCING A CATHODOLUMINESCENT PHOSPHOR WHICH COMPRISESTHE STEPS OF (A) PREPARING A FIRST HOT ACIDIC SOLUTION CONTAININGAPPROXIMATELY 99 MOLE PERCENT YTTRIUM OXIDE AND APPROXIMATELY 1 MOLEPERCENT OF A DOPANT SELECTED FROM THE GROUP CONSISTING OF MAGNESIUM,CADMIUM, COPPER AND THORIUM, (B) REACTING SAID FIRST SOLUTION WITH ASECOND HOT SOLUTION OF OXALIC ACID, (C) ALLOWING THE REACTANTS TO COOLTO PRODUCE AN OXALATE PRECIPITATE OF YTTRIUM AND A DOPANT SELECTED FROMSAID GROUP, AND (D) HEATING SAID OXALATE PRECIPITATE TO CONVERT THEOXALATE TO AN OXIDE OF YTTRIUM INCORPORATING AN ELEMENT FROM SAID GROUP,THE RESULTANT PHOSPHOR EXHIBITING A BLUE-WHITE CATHODOLUMINESCENCE.